When you open a folder in CotEditor, it is displayed as a folder document with a file browser in the sidebar. This allows you to work with multiple files in the folder from a single window.
Note: The sidebar containing the file browser is available only when you open a folder in CotEditor. If you open a regular file, you can’t add the sidebar to that window later.
Go to the CotEditor app 
on your Mac.
Do one of the following:
Choose File > Open, choose a folder, then click Open.
Drag a folder onto the CotEditor app icon.
The folder opens as a folder document, and its contents appear in the sidebar.
You can browse files and subfolders in the folder browser in the sidebar. When you click a file, the document opens in the same window, allowing you to switch between multiple files in the folder while editing.
Note: Window tabs aren’t available for folder documents.
Tip: You can switch the display of hidden files in the file browser by using the Show Hidden Files command in the contextual menu. For details, see Open hidden files.
In the CotEditor app on your Mac, open a folder document.
Enter a search term in the filter field at the bottom of the file browser.
To end the filter state, click the Delete button in the filter field to empty it.
When you select a non-text file in the file browser, the window shows information and a preview of the document. The displayed content varies depending on the file type.
In the document preview, you can do the following:
Open in an external editor: Click “Open in External Editor,” or click the down arrow in the button to choose an editor to open the file in an external app. If you click “Open in External Editor,” the file opens in the default app for the file type.
Open as plain text: Click “Open as Plain Text” to force the file to be interpreted as plain text and open it in a CotEditor window.
If the file is an image, movie, or PDF: You can preview the file contents.
If the file is an alias or symlink: You can open the original item. For details, see Open aliases or symlinks.